The present invention relates to a line head in which a plurality of light emission elements are arrayed is used as an exposer for forming a latent image to be developed, which is incorporated in an image forming apparatus using an electrophotographic process.
Japanese Patent Publication No. 4-363264A discloses a line head in which a plurality of electroluminescence (EL) elements are arrayed to form a single line, and teaches that an interval between pulses for driving the elements to emit light is controlled so as to elongate the life of the elements. Japanese Patent Publication No. 3-101366A discloses a line head in which a number of EL elements are arrayed to form a single line, one auxiliary pulse for driving all the elements to emit light is necessarily incorporated irrespective of main driving pulses in each primary scanning operation, so that predetermined light intensity can be obtained in a short time even when the devices have not emitted light for a long time.
FIG. 20 shows one example of a time-to-luminance characteristic of an EL element. The abscissa is used for a time (minutes) and the ordinate is used for luminance. As is shown, luminance decreases with an elapse of time since the EL element has activated (by about 20% in five minutes). The luminance is restored if the EL element is deactivated for a predetermined time period to decrease the element temperature, and is activated again. To simplify the explanation, the deactivated period is omitted from FIG. 20.
Each of the line heads disclosed in the above publications is provided with a single array of the EL elements. Hence, if such a line head is subjected to continuous activation for a long time period, the luminance decreases as shown in FIG. 20, thereby deteriorating the quality of an image to be obtained. In addition, since the degree of the luminance decrease in one element is actually different from another element, the deterioration of the obtained image quality becomes remarkable after the long-term activation. Further, since the life of the EL element is shortened by such a continuous activation, the replacement of the line head is hastened.
Moreover, at the occurrence of an unexpected event due to a failure in any of the light emission elements or the like, the line head has to be replaced, which poses a problem that the replacement is tedious and the cost is thereby increased.